This invention relates to radio determination by artificial earth satellites, and more particularly to a method of position surveillance with one active ranging satellite and an independent satellite transmitting timing signals. For a position fix with altitude, an additional satellite transmitting timing signal is needed.
To locate a maritime or land vehicle on the surface of the earth by range measurements, two satellites are required. Radio signals travel at the velocity of light, and range is determined from the free space propagation time delay of a timing or ranging signal transmitted by a satellite and received on the vehicle. Knowing the satellite locations, a line of position for the vehicle from each satellite can be computed, the position fix being at the intersection of the two lines of position. The fact that two satellites are required is frequently given as an objection when it is suggested that communications satellites be used for radio determination, which is defined as position fixing by the use of radio signals. The present invention overcomes that objection by requiring only one active ranging (and communications) satellite. The function of the second satellite is fulfilled by any other independent satellite that is transmitting timing signals, such as a standard time distribution satellite, and is in orbit for reasons that are entirely independent of the surveillance system. It does not have to receive and retransmit any signals specifically for the position fixing system.
The invention makes it attractive to implement position surveillance, i.e., tracking the position of a vehicle or craft at a fixed earth station, at an early date with existing or contemplated satellites. Some applications are monitoring the positions of oil tankers and other vessels with dangerous cargoes to help prevent mishaps and protect the enviroment; aid the effective monitoring of foreign vessels within the newly assigned 200 mile limit, transoceanic aircraft surveillance for traffic control; aid commercial shipping navigation and ship position monitoring from the shore; and land mobile vehicle position monitoring, as for law enforcement. The technique can provide worldwide service, high accuracy can be achieved, and costs are modest as compared to other proposed systems.
Navigation and position surveillance by another method involving measurement of the time of arrival of signals from two timing satellites by an onboard crystal clock, with correction of the clock by an occasional two-way active range measurement, is disclosed and claimed in concurrently filed application Ser. No. 842,402 by the inventor, assigned to the same assignee as this invention. Various active and passive one-way and two-way ranging techniques are also described in the inventor's U.S. Pat. No. 3,384,891, which was granted on May 21, 1968 and is assigned to the same assignee.